In recent years, as color television receivers have been increased in size, many color television receivers are provided with gradation correction devices such as a black level correction circuit for expanding the gradation of a luminance signal of an image toward the black side or a gamma correction circuit, in order to display the image more clearly and dynamically. Further, in order to improve the accuracy of gradation correction, digital signal processing techniques have increasingly been employed.
FIG. 18 is a block diagram illustrating a conventional gradation correction device for a luminance signal, which is disclosed in Japanese Published Patent Application No. Hei.4-37263. The gradation correction device shown in FIG. 18 is provided with a maximum luminance level detection circuit 1401; a comparison circuit 1402 for comparing the output of the maximum luminance level detection circuit 1401 with a reference value; a gamma correction circuit 1403 for performing gamma correction on the basis of the output of the comparison circuit 1402; a matrix circuit 1404 for supplying R, G, and B signals to a cathode ray tube (CRT); and the CRT 1405.
Hereinafter, the operation will be described. The maximum luminance level detection circuit 1401 receives an output luminance signal B, detects a maximum luminance level C within one field of the output luminance signal B, and outputs it to the comparison circuit 1402.
The comparison circuit 1402 compares the maximum luminance level C with a reference luminance level D which is supplied from the outside, and outputs a comparison signal E to the gamma correction circuit 1403.
The gamma correction circuit 1403 controls a gamma correction gain according to the value of the comparison signal E, performs gamma correction on an input luminance signal A, and outputs an output luminance signal B having a gamma correction characteristic as shown in FIG. 19(a) or 19(b). That is, when the maximum luminance level C is larger than the reference luminance level D, gamma correction for suppressing the white-side gradation is carried out as shown in FIG. 19(a), and when the maximum luminance level C is smaller than the reference luminance level D, inverse gamma correction for expanding the white-side gradation is carried out as shown in FIG. 19(b), whereby an output luminance signal B is obtained to be outputted to the matrix circuit 1404.
The matrix circuit 1404 performs a matrix operation on the output luminance signal B and a chrominance signal Cin to obtain output signals R (red), G (green), and B (blue), and outputs these signals to the CRT 1405.
Then, the CRT 1405 is driven by the output signals R, G, and B, thereby obtaining a gradation-corrected image.
In the conventional gradation correction device constituted as described above, comparison between the maximum luminance level C outputted from the maximum luminance level detection circuit 1404 and the reference luminance level D is performed by the comparison circuit 1402, and the gamma correction gain is controlled according to the comparison signal E, whereby gradation correction is carried out by analog feedback control such that the output luminance signal B becomes equal to the reference luminance signal D. However, as described above, in a rising tide of digitization of video signal processing in recent years, there is a necessity of shifting the feedback control which has been carried out in analog fashion by the conventional gradation correction device, to feedback control in digital fashion.
The present invention is made in view of the existing situation described above and has for its object to provide a gradation correction device that can perform gradation correction using feedback processing even in digital fashion, and that can perform control with high accuracy.